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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Ordered Particle Packing in Dense TRISO/SiC Fuel Elements and Preliminary Assessment of Neutronic and Thermomechanical Characteristics

Detailed analysis of the particle distribution in Transformational Challenge Reactor fuel elements indicates that particle packing is not random; instead, it follows a relatively ordered structure near fuel element surfaces. Discrete particle neutronic simulations indicate that the core reactivity is not impacted when assuming homogenization of particles with the silicon carbide matrix. However, the neutronic power distribution resulting from the ordered packing structure indicates that the highest-power particles reside at the top and bottom of the fuel elements and nearest the YH 1.85 moderator rods. The power distribution results were applied to thermo-mechanical simulations using mesh-based power distributions. Previous results indicated high stress at the bottom of the fuel element, where packing is most ordered. Additionally, to reduce this stress concentration, additively manufactured protrusions were added to the bottom of a test fuel element to disrupt dense particle packing. These protrusions reduced the overall power peaking, but the thermomechanical simulations did not indicate a significant change in the fuel element’s maximum stress or failure probability.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2026

Microstructural Engineering of Cu-Rich Nanoprecipitate formation in NiCoFeCrCu0.12 High-Entropy Alloy via Severe Plastic Deformation for Enhanced Irradiation Tolerance

This study demonstrates a defect-engineering approach for controlling Cu-rich precipitates in FeNiCrCoCu0.2 high-entropy alloys (Cu-HEAs), delivering a novel pathway for next-generation nuclear reactor materials with superior irradiation resistance. This work establishes that severe plastic deformation (SPD) processing via Shear Assisted Processing and Extrusion (ShAPE) and Friction Stir Layer Deposition (FSLD) creates dense dislocation networks and subgrain boundaries that fundamentally alter precipitation behavior under identical thermal treatments. Atom probe tomography (APT) indicates that SPD produces a metastable, atomically homogeneous solid solution that, upon moderate heat treatment (500°C/10 hour), develops remarkedly stronger Cu clustering than the as-cast counterpart. High-temperature exposure (800°C/100 h) produces near-pure Cu precipitates (~90 at% Cu) with significantly enhanced defect-sink efficacy in SPD-processed alloys: precipitate sizes of 50-60 nm and number densities of 2.7-3.8 × 10¹7 m?³, compared to 89 nm and 0.44 × 10¹7 m?³ in as-cast materials. Collectively, the findings establish defect-mediated precipitation control as a scalable, high-impact route to tailor sink density and distribution in HEAs, enabling microstructures optimized for irradiation tolerance and mechanical robustness in nuclear reactor environments.

Meher, Subhashish↗
NASA NTRS2014

Formation of Fe/mg Smectite Under Acidic Conditions from Synthetic Adirondack Basaltic Glass: an Analog to Fe/mg Smectite Formation on Mars

Smectite has been detected as layered material hundreds of meters thick, in intracrater depositional fans, in plains sediments, and deposits at depth on Mars. If early Mars hosted a dense CO2 atmosphere, then extensive carbonate should have formed in the neutral/alkaline conditions expected for smectite formation. However, large carbonate deposits on Mars have not been discovered. Instead of neutral to moderately alkaline conditions, early Mars may have experienced mildly acidic conditions that allowed for Fe/Mg smectite formation but prevented widespread carbonate formation. The objective of this work is to demonstrate that Fe(II)/Mg saponite and nontronite can form in mildly acidic solutions (e.g., pH 4). Synthetic basaltic glass (< 53 microns) of Adirondack rock class composition was exposed to pH 4 (acetic acid buffer) and N2 purged (anoxic) solutions amended with 0 and 10 mM Mg or Fe(II). Basaltic glass in these solutions was heated to 200 C in batch reactors for 1, 7, and 14 days. X-ray diffraction analysis of reacted materials detected the presence of phyllosilicates as indicated by a approx. 15.03-15.23Angstroms (001) peak. Smectite was confirmed as the phyllosilicate after treatments with glycerol and KCl and heating to 550 C. Trioctahedral saponite was confirmed by the presence of a 4.58 to 4.63 Angstroms (02l) and 1.54Angstroms (060) peaks. Saponite concentration was highest, as indicated by XRD peak intensity, in the 10 mM Mg treatment followed by the 0 mM and then 10 mM Fe(II) treatments. This order of sapontite concentration suggests that Fe(II) additions may have a role in slowing the kinetics of saponite formation relative to the other treatments. Nontronite synthesis was attempted by exposing Adirondack basaltic glass to pH 4 oxic solutions (without N2 purge) at 200 C for 14 days. X-ray diffraction analysis indicated that mixtures of trioctahedral (saponite) and dioctahedral (nontronite) may have formed in these experiments based on the 02l and 060 peaks. Moessbauer analysis coupled with future experiments are planned to verify if nontronite can be formed under mildly acidic and oxic conditions. Results of this work demonstrate that acidic conditions could have occurred on an early Mars, which allowed for smectite formation but inhibited carbonate formation.

Sutter, B.↗
NASA NTRS1975

Interstellar molecular abundances toward omicron Persei

Extensive scanning of the B1 III star omicron Per has been done with the Copernicus telescope-spectrometer in an effort to determine column densities or meaningful upper limits for several molecules expected to be relatively abundant in H I regions of moderate density. The combination of radio H II, OH, and CO emission-line data with the optical absorption measurements indicates that omicron Per is embedded in the near edge of a dense molecular cloud and that the material seen in front of the star must be close to it. Molecules detected in ultraviolet absorption are H2, HD, CO, and possibly OH. The CO rotational temperature is between 2.2 and 3.8 K. The observed column densities and upper limits from Copernicus and ground-based data are compared with recent models for molecular formation in diffuse clouds.

Snow, T. P., Jr.↗
NASA NTRS1990

Interstellar grain chemistry and organic molecules

The detection of prominant infrared absorption bands at 3250, 2170, 2138, 1670 and 1470 cm(-1) (3.08, 4.61, 4.677, 5.99 and 6.80 micron m) associated with molecular clouds show that mixed molecular (icy) grain mantles are an important component of the interstellar dust in the dense interstellar medium. These ices, which contain many organic molecules, may also be the production site of the more complex organic grain mantles detected in the diffuse interstellar medium. Theoretical calculations employing gas phase as well as grain surface reactions predict that the ices should be dominated only by the simple molecules H2O, H2CO, N2, CO, O2, NH3, CH4, possibly CH3OH, and their deuterated counterparts. However, spectroscopic observations in the 2500 to 1250 cm(-1)(4 to 8 micron m) range show substantial variation from source reactions alone. By comparing these astronomical spectra with the spectra of laboratory-produced analogs of interstellar ices, one can determine the composition and abundance of the materials frozen on the grains in dense clouds. Experiments are described in which the chemical evolution of an interstellar ice analog is determined during irradiation and subsequent warm-up. Particular attention is paid to the types of moderately complex organic materials produced during these experiments which are likely to be present in interstellar grains and cometary ices.

Allamandola, L. J.↗
DOE OSTI2025

Parity-Doubled Nucleons Can Rapidly Cool Neutron Stars

In confined hadronic matter, the spontaneous breaking and restoration of chiral symmetry can be described by considering nucleons, 𝑁 + ⁡(939), and excited states of opposite parity, 𝑁 − ⁡(1535). In a cold, dense hadronic phase where chiral symmetry remains spontaneously broken, direct Urca decay processes involving the 𝑁 − are possible, e.g., 𝑁 − → 𝑁 + + 𝑒 − + $\bar{𝜈}_𝑒$. Here, we show that at low temperature and moderate densities, because the 𝑁 − are much heavier than the 𝑁 + , such cooling dominates over standard 𝑁 + direct Urca processes. This provides a strong astrophysical signature of the pattern of chiral symmetry restoration in neutron stars.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
NASA NTRS1985

Detection of [SiLL] (34.8 micron) emission in Orion-KL: A measurement of the silicon abundance in dense interstellar gas

The first detection of the ground state fine structure transition of Si+ at a rest wavelength determined to be 34.815 + or - 0.004 micron are reported. These observations were obtained with the facility spectrometer on NASA's Kuiper Airborne Observatory. A 6' NW-SE strip scan across the Orion-KL region shows SiII emission from both the extended photodissociation region surrounding theta 1 Ori C and from the shocked gas NW of BN-KL. The inferred gas-phase silicon elemental abundance relative to hydrogen in the dense 10 to the 5/cc primarily neutral photodissociation region is approximately 2.6 x to the -6, a factor of 0.075 times the solar value and 3.4 times greater than the abundance in the moderate density approx. 10 to the 3/cc cloud toward zeta Oph. The silicon abundance in the shocked gas is approximately solar, indicating that any pre-existing grains have been destroyed in the shock wave or that the preshock gas carries a near solar abundance of gas phase silicon. The shock-excited SiII (34.8 micron) emission may arise from shocked wind material in the outflow around IRc2, with wind velocities approx. 100 km/s.

Haas, M. R.↗
NASA NTRS1986

Detection of (Si II) (34.8 micron) emission in Orion-KL - A measurement of the silicon abundance in dense interstellar gas

The first detection of the ground state fine structure transition of Si+ at a rest wavelength determined to be 34.815 + or - 0.004 micron are reported. These observations were obtained with the facility spectrometer on NASA's Kuiper Airborne Observatory. A 6' NW-SE strip scan across the Orion-KL region shows Si II emission from both the extended photodissociation region surrounding theta 1 Ori C and from the shocked gas NW of BN-KL. The inferred gas-phase silicon elemental abundance relative to hydrogen in the dense 10 to the 5/cc primarily neutral photodissociation region is approximately 2.6 x 10 the -6, a factor of 0.075 times the solar value and 3.4 times greater than the abundance in the moderate density aprox. 10 to the 3/cc cloud toward Zeta Oph The silicon abundance in the shocked gas is approximately solar, indicating that any pre-existing grains have been destroyed in the shock wave or that the preshock gas carries a near solar abundance of gas phase silicon. The shock-excited Si II (34.8 micron) emission may arise from shocked wind material in the outflow around IRc2, with wind velocities approx. 100 km/s.

Haas, M. R.↗
NASA NTRS1974

Tropical cyclone rainfall as measured by the Nimbus 5 electrically scanning microwave radiometer

A selected group of 1973 North Pacific Ocean tropical cyclones was studied by using data from the Nimbus 5 Electrically Scanning Microwave Radiometer (ESMR), the Temperature-Humidity Infrared Radiometer (THIR), NOAA-2 and USAF DMSP imageries. From the unique combination of infrared, visible, and microwave data, it was possible during various stages of storm development to differentiate between dense cirrus outflow and rain areas, to identify centers of circulation and areas of low-level moisture, and by the use of a theoretical model to estimate semi-quantitatively areas of light, moderate, and heavy rainfall rates.

Allison, L. J.↗
NASA NTRS2001

The AstroBiology Explorer (ABE) MIDEX Mission Concept

The Astrobiology Explorer (ABE) is a MIDEX mission concept under study at NASA's Ames Research Center in collaboration with Ball Aerospace & Technologies, Corp. ABE will conduct IR spectroscopic observations to address important problems in astrobiology, astrochemistry, and astrophysics. The core observational program would make fundamental scientific progress in understanding the distribution, identity, and evolution of ices and organic matter in dense molecular clouds, young forming stellar systems, stellar outflows, the general diffuse ISM, HII regions, Solar System bodies, and external galaxies. The ABE instrument concept includes a 0.6 m aperture Cassegrain telescope and two moderate resolution (R = 2000-3000) spectrographs covering the 2.5-16 micron spectral region. Large format (1024x 1024 pixel or larger) IR detector arrays and bandpass filters will allow each spectrograph to cover an entire octave of spectral range or more per exposure without any moving parts. The telescope will be cooled below 50K by a cryogenic dewar shielded by a sunshade. The detectors will be cooled to approximately 8K. The optimum orbital configuration for achieving the scientific objectives of the ABE mission is a low background, 1 AU Earth driftaway orbit requiring a Delta II launch vehicle. This configuration provides a low thermal background and allows adequate communications bandwidth and good access to the entire sky over the approximate 1-2 year mission lifetime.

Ennico, Kimberly↗
NASA NTRS1991

Probing the core of Cepheus A - Millimeter and submillimeter observations

Moderate and high angular resolution (40-7 arcsec) maps are presented of the core of the Cepheus A star-forming region using CS J = 3-2 and J = 7-6 emission, which traces the dense gas component of the cloud core, and using far-infrared (450 and 800 microns) continuum emission tracing the warm dust component. Three regimes in the core are traced by these observations: (1) a small (about 0.14 pc), nearly circular central core of high density (1-10 x 10 to the 6th/cu cm) and temperature (30-100 K) containing at least 25 percent of the mass and which contains the active early-type star formation; (2) an extended (0.5 x 0.25 pc), NE-SW oriented core of mass 200-300 solar masses, temperature 30-40 K, and average density nH2 of about 10 to the 5th/cu cm and which, together with the central core, contains 60-80 percent of the total core mass; and (3) an extended core envelope of dimensions 0.5 x 0.85 pc oriented primarily north-south, and a lower density. The velocity structure of the core suggests that it is being disrupted by the high-velocity winds driving the molecular outflow and is not due to a rotating circumstellar disk.

Moriarty-Schieven, G. H.↗
NASA NTRS2001

Organic Signature of Dust from the Interstellar Medium (ISM)

Dust in the ISM carries an "organic" signature in form of a distinct group of C-H stretching bands, both in emission and absorption, around 3.4 micrometers. These bands agree with the symmetrical and asymmetrical C-H stretching vibrations of aliphatic -CH2- entities and are thought to be associated with organic molecules on the surface of dust grains. We show that this interpretation is inconsistent with laboratory experiments. Synthetic MgO and natural olivine single crystals, grown from a CO/CO2/H2O-saturated melt, exhibit the same C-H stretching bands but those bands are clearly associated with C-H entities inside the dense mineral matrix. The multitude of C-H stretching bands suggests that the C-H bonds arise from polyatomic C(sub n) entities. We heated the MgO and olivine crystals to temperatures between 550-1000 K to pyrolyze the C-H bonds and to cause the C-H stretching bands to disappear. Upon annealing at moderate temperatures between 300-390 K the C-H stretching bands reappear within a few days to weeks. The C-H stretching band intensity increases linearly with the square root of time. Thus, while the pyrolysis broke the C-H bonds and caused the H to disperse in the mineral matrix, the H atoms (or H2 molecules) are sufficiently mobile to return during annealing and reestablish the C-H bonds. Dust grains that condense in a gas-laden environment (outflow of late-stage stars or in dense molecular clouds) probably incorporate the same type of Cn-H entities. Imbedded in and in part bonded to the surrounding mineral matrix, the Cn-H entities display C-H stretching bands in the 3.4 micrometer region, but their lower frequency librational modes are so strongly coupled to the lattice modes that they broaden excessively and thus become unobservable.

Freund, Friedemann↗
NASA NTRS1989

Further evidence for a circumstellar disk around PV Cephei

Spectra of PV Cephei from 2 to 4 microns reveal a moderately strong 3 micron ice-absorption feature. Normally seen in objects with high visual obscuration, A(V) greater than 10, this absorption in an object with A(V) = 3.5-5.0 suggests the existence of a dense circumstellar disk. This paper discusses the evidence for a disk from this data and other wavelength regions, the possible origin of the 3-micron feature in PV Cephei, and the relationship between this object and other young stellar objects.

Van Citters, G. W., Jr.↗
NASA NTRS2002

The AstroBiology Explorer (ABE) MIDEX Mission Concept: Identifying Organic Molecules in Space

The Astrobiology Explorer (ABE) is a MIDEX mission concept, currently under Concept Phase A study at NASA's Ames Research Center in collaboration with Ball Aerospace & Technologies, Corp., and managed by NASA's Jet Propulsion Laboratory. ABE will conduct infrared spectroscopic observations to address important problems in astrobiology, astrochemistry, and astrophysics. The core observational program would make fundamental scientific progress in understanding the distribution, identity, and evolution of ices and organic matter in dense molecular clouds, young forming stellar systems, stellar outflows, the general diffuse ISM, HII regions, Solar System bodies, and external galaxies. The ABE instrument concept includes a 0.6 m aperture Ritchey-Chretien telescope and three moderate resolution (R = 2000-3000) spectrometers together covering the 2.5-20 micron spectral region. Large format (1024 x 1024 pixel) IR detector arrays will allow each spectrometer to cover an entire octave of spectral range per exposure without any moving parts. The telescope will be cooled below 50 K by a cryogenic dewar shielded by a sunshade. The detectors will be cooled to approx. 7.5 K by a solid hydrogen cryostat. The optimum orbital configuration for achieving the scientific objectives of the ABE mission is a low background, 1 AU Earth driftaway orbit requiring a Delta II launch vehicle. This configuration provides a low thermal background and allows adequate communications bandwidth and good access to the entire sky over the approx. 1.5 year mission lifetime.

Ennico, Kimberly↗
NASA NTRS1994

Temporal changes in sarcomere lesions of rat adductor longus muscles during hindlimb reloading

Focal sarcomere disruptions were previously observed in adductor longus muscles of rats flown approximately two weeks aboard the Cosmos 1887 and 2044 biosatellite flights. These lesions, characterized by breakage and loss of myofilaments and Z-line streaming, resembled damage induced by unaccustomed exercise that includes eccentric contractions in which muscles lengthen as they develop tension. We hypothesized that sarcomere lesions in atrophied muscles of space flow rats were not produced in microgravity by muscle unloading but resulted from muscle reloading upon re-exposure to terrestrial gravity. To test this hypothesis, we examined temporal changes in sarcomere integrity of adductor longus muscles from rats subjected to 12.5 days of hindlimb suspension unloading and subsequent reloading by return to vivarium cages for 0, 6, 12, or 48 hours of normal weightbearing. Our ultrastructural observations suggested that muscle unloading (0 h reloading) induced myofibril misalignment associated with myofiber atrophy. Muscle reloading for 6 hours induced focal sarcomere lesions in which cross striations were abnormally widened. Such lesions were electron lucent due to extensive myofilament loss. Lesions in reloaded muscles showed rapid restructuring. By 12 hours of reloading, lesions were moderately stained foci and by 48 hours darkly stained foci in which the pattern of cross striations was indistinct at the light and electron microscopic levels. These lesions were spanned by Z-line-like electron dense filamentous material. Our findings suggest a new role for Z-line streaming in lesion restructuring: rather than an antecedent to damage, this type of Z-line streaming may be indicative of rapid, early sarcomere repair.

Non-NASA Center↗
NASA NTRS1994

Disintegration of Dust Aggregates in Interstellar Shocks and the Lifetime of Dust Grains in the ISM

Interstellar grains are destroyed by shock waves moving through the ISM. In fact, the destruction of grains may be so effective that it is difficult to explain the observed abundance of dust in the ISM as a steady state between input of grains from stellar sources and destruction of grains in shocks. This is especially a problem for the larger grains. Therefore, the dust grains must be protected in some way. Jones et al. have already considered coatings and the increased post-shock drag effects for low density grains. In molecular clouds and dense clouds, coagulation of grains is an important process, and the largest interstellar grains may indeed be aggregates of smaller grains rather than homogeneous particles. This may provide a means to protect the larger grains, in that, in moderate velocity grain-grain collisions in a shock the aggregates may disintegrate rather than be vaporized. The released small particles are more resilient to shock destruction (except in fast shocks) and may reform larger grains later, recovering the observed size distribution. We have developed a model for the binding forces in grain aggregates and apply this model to the collisions between an aggregate and fast small grains. We discuss the results in the light of statistical collision probabilities and grain life times.

Dominik, C.↗
DOE OSTI2020

Equation of State of CO 2 Shock Compressed to 1 TPa

Equation-of-state (pressure, density, temperature, internal energy) and reflectivity measurements on shock-compressed CO 2 at and above the insulating-to-conducting transition reveal new insight into the chemistry of simple molecular systems in the warm-dense-matter regime. CO 2 samples were precompressed in diamond-anvil cells to tune the initial densities from 1.35 g/cm 3 (liquid) to 1.74 g/cm 3 (solid) at room temperature and were then shock compressed up to 1 TPa and 93 000 K. Lastly, variation in initial density was leveraged to infer thermodynamic derivatives including specific heat and Gruneisen coefficient, exposing a complex bonded and moderately ionized state at the most extreme conditions studied.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Algorithm 1049: The Delaunay Density Diagnostic

Accurate approximation of a real-valued function depends on two aspects of the available data: the density of inputs within the domain of interest and the variation of the outputs over that domain. There are few methods for assessing whether the density of inputs is sufficient to identify the relevant variations in outputs—i.e., the “geometric scale” of the function—despite the fact that sampling density is closely tied to the success or failure of an approximation method. In this article, we introduce a general purpose, computational approach to detecting the geometric scale of real-valued functions over a fixed domain using a deterministic interpolation technique from computational geometry. The algorithm is intended to work on scalar data in moderate dimensions (2–10). Our algorithm is based on the observation that a sequence of piecewise linear interpolants will converge to a continuous function at a quadratic rate (in L 2 norm) if and only if the data are sampled densely enough to distinguish the feature from noise (assuming sufficiently regular sampling). We present numerical experiments demonstrating how our method can identify feature scale, estimate uncertainty in feature scale, and assess the sampling density for fixed (i.e., static) datasets of input–output pairs. Finally, we include analytical results in support of our numerical findings and have released lightweight code that can be adapted for use in a variety of data science settings.

97 MATHEMATICS AND COMPUTING↗